Effective Objective－C 2.0 第5条：用枚举表示状态、选项、状态码

简单枚举类型

enum EOCConnectionState {    EOCConnectionStateDisconnected,    EOCConnectionStateConnecting,    EOCConnectionStateConnected,}typedef enum EOCConnectionState EOCConnectionState;

这样就可以简写的EOCConnectionState来代替完整的enum EOCConnectionState。

指定底层数据类型

enum EOCConnectionState : NSInteger {/* ... */}

或者向前声明：

enum EOCConnectionState : NSInteger;

彼此组合的枚举

各选项之间可以通过”按位或操作符”。

enum UIViewAutoresizing {    UIViewAutoresizingNone = 0,    UIViewAutoresizingFlexibleLeftMargin = 1 << 0,    UIViewAutoresizingFlexibleWidth = 1 << 1,    UIViewAutoresizingFlexibleRightMargin = 1 << 2,    UIViewAutoresizingFlexibleTopMargin = 1 << 3,    ...}

Foundation框架辅助宏

这些宏具有向后兼容能力。

typedef NS_ENUM(NSUInteger, EOCConnectionState) {    EOConnectionStateDisconnected,    ...}

typedef NS_OPTIONS(NSUInteger, EOCPermittedDirection) {    EOCPermittedDirectionUp = 1 << 0,    ...}

用枚举定义定义状态机，最好不要有default分支。

switch(_currentState) {    case EOCConnectionStateDisconnected:        break;    case EOCConnectionStateConnecting:        break;    case EOCConnectionStateConnected:        break;}